Air purification



Nov. 20, 1962 Filed May 25, I958 GRAINS OF WATER PER LB. F nrv A? INEQU/LBR/UM Y/TH JOLUT/ON G. A. KELLEY AIR PURIFICATION 2 Sheets-Sheet 1L/TH/Ll/"I CHLORIDE CHART 0 10 2o 5 CONCENTRATION OF SOLUTION PERCENT BYW67.

INVENTOR. 5.14. KE'LL 5v United rates Patent 3,065,943 AIR PUREI'ICATEON Gilbert A. Kelley, Toledo, Ghio, assignor, by mesneassignments, to Midland-Ross Corporation, Cleveland, Ohio, a corporationof Ohio Filed May 23, 1958, Ser. No. 737,993 Claims. (1. 21--53) Thisinvention relates to the purification and sanitization of air by the useof water solutions of lithium chloride, and to a simple apparatustherefor which requires only a pump, and no controls, to operatecontinuously and eliectively under varying conditions of the airtreated. This application is a continuation-in-part of my applicationSerial No. 354,289, filed May 11, 1953, now abandoned.

For a consideration of what I believe to be novel and my invention,attention is directed to the following portion of this specification andthe drawing and concluding claims thereof.

In the drawing:

FIGURE 1 is a diagrammatic representation of apparatus according to, andfor practicing this invention.

FIGURE 2 is a chart of solubility lithium chloride in water.

FIGURE 3 is a diagrammatic representation of alternate apparatus.

FIGURE 4 is a chart showing the relation of capture of micro-organismsto the number of rows of wetted coils, or the thickness of wettedfilter.

Most air carried bacteria, molds and the like microorganisms, cannotexist in even relatively weak solution of lithium chloride. For example,at room temperatures, about 70 F, the Bacillus subtilis, a Gram-positiveorganism, grows in a 2.2% solution of lithium chloride, but does notgrow in 2.3% solution thereof. E. coli, a Gramnegative organism, willgrow in a 2.7% solution of lithium chloride, but will not grow in a 2.8%solution thereof. These organisms are commonly used as a standards inthis type of investigations in the :art.

Lithium chloride is hygroscopic, and if left for an extended period oftime in contact with atmospheric air, it will reach an equilibriumconcentration of salt in the absorbed moisture according to the chart ofFIGURE 2. In the winter time, in a room at 70 F. and having 11 grainsper pound absolute moisture content, the concentration of lithiumchloride in equilibrium therewith is about 45 and on a warm summer day,early in the morning, in an 80 F. room the equilibrium concentration oflithium chloride may be as low as about 12%; and in the afternoon of thesame day the concentration might rise to 26%. The range of equilibriumconcentration to be expected in an enclosed living space over a yearstime would be about from 12% to 45%.

If a solution of a fixed weight of lithium chloride salt in water werecontinuously recirculated over a contactor in contact with a stream ofroom air, the volume of solution circulating would vary up to about 3%times smallest volume found over a years time due to the variation ofwater in the solution. Thus an air contacting unit operable with asolution content of 1.0 gallon of solution and having an operablecapacity of up to about 3.75 gallons could be filled with 3% gallons of12% solution, or one gallon of 45% solution, and it would thereafteralways contain from one to 3% gallons of solution depending upon thetemperature and moisture content of the air contacting the solution.

According to this invention, an unregenerated equilibrium solution oflithium chloride, i.e. a water solution thereof in equilibrium withatmosphere air, is continuously recirculated over an air-to-liquidcontactor of a type used in air conditioning, and a stream ofatmospheric air is circulated therethrough in contact with the solution.

Air borne micro-organisms and suspended solids such as microscopicparticles of sand, metal, ashes, soot, fiber, chemicals, animal andvegetable matter, together with bacteria, mold spores, pollens and thelike are carried into contact with the solution. The cont-actor has abacteriostatic action, and the air borne micro-organisms in the airstream leaving the contactor depends primarily upon the efiiciency ofcontacting the air stream with the solution in the unit. Generallyspeaking, removal of air borne micro-organisms parallels the moistureremoval efficiency of the same contactor when used with concentrated,hygroscopic solution, and an air conditioner of a given design usingconcentrated hygroscopic solution which removes 95% of excess moisturefrom the air over the equilibrium humidity for the solution. may remove95 of the air borne bacteria, molds, and the like from the air in asingle pass through the contactor. Obviously, if air from a room iscontinuously recirculated over an equilibrium solution of lithiumchloride, the air in the room will be very rapidly depleted of itsbacteria, molds, and the like, and will in time become sanitary, orsubstantially sterile, except for possible new contamination of the air.

This invention was tested by passing air from a sealed room through alithium chloride solution to air contactor, and back into the room usingSerratia marcescens as a test bacteria because it was not common to thearea, is extremely small (about .25 micron), and because it providescharacteristic red colonies on agar. The room volume was 981 cubic feet,substantially cubical, and tests were made by placing Petri dishescontaining nutrient agar and Czapek media in the room on the floor, forvarying periods, usually about 15 minutes, or in an air stream to orfrom the contacting unit for about 10 minutes, and the dishes were thenincubated at 25 C. for three days, then bacteria and mold colonies werecounted.

The effectiveness of the unit in the test procedure was tested byspraying Serratia marcescens, or red bacteria, into the inlet stream tothe unit. The red count therefrom varied from 278 to 304 at the inlet to10 to 39 at the outlet in one test. At a recirculation rate of 650 cubicfeet per minute, test dishes exposed 15 minutes after spraying thebacteria into the room showed no airborne bacteria in the room, thoughthe bacteria remained air-borne for three hours after spraying when theair circulation to the contactor was turned olf.

When tests were run with a technician in, or entering, the room, it waspractically impossible to remove all normal air carried bacteria andmolds, but the count thereof decreased rapidly to a very small number.By setting two contacting units in series in an air stream, theefficiency of bacteria removal is increased from about to about 98%under similar conditions. When the solution is only sprayed across anair stream without the use of an air-liquid contact surface, theeffectiveness of the process is noticeably reduced, which parallelsdehumidifying experience in air conditioning.

The mechanism by which air-borne micro-organisms, dust and the like areabsorbed is different than that by which moisture is absorbed by ahyrgoscopic solution, and is beginning to be understood. In humiditycontrol apparatus using hygroscopic solutions, it can be shown that theapproach to equilibrium of the humidity in the treated air (to theequilibrium humidity for the solution) is a function of wetted surfacefor contacting the air stream and contact time. In removal of air-bornemicro-organisms, other factors are more important. It is believed atpresent that an impingement effect is required to initially capturepractical fraction of dust, micro-organisms and the like from the airstream.

aosaoas An equilibrium concentration of lithium chloride will have abacteriostatic action over a wide range of atmospheric conditions. Thevalue of a really effective, yet simple, bacteriostatic air treatingdevice in producing near-sterile air is very great where it is desiredto avoid contamination and spoilage of food, mold growth, spread ofdisease and the like.

A simple apparatus for sterilization of air in an enclosed space isillustrated in FIGURE 1 of drawing where an air treating unit comprisinga casing 10 is inserted in an air .duct 11 through which air is causedto flow by a fan mechanism 12. An air to liquid contactor 13 is disposedin the casing, and an equilibrium lithium chloride solution iscirculated from a sump 14 in the lower portion of the casing through aconduit 15 by a pump 16 to nozzles 17 in the upper portion of thecontactor. The solution flows by gravity over the contactor in intimatecontact with air flowing therethrough. Air carried bacteria in the airentering the cont-actor are largely removed; only about 1 to 10 percentof the bacteria by count pass through the contactor in a single pass ofair, and upon recirculation of air from an enclosure through thecontactor, the bacteria count of the air in the enclosure is reduced tosubstantially Zero.

Alternate apparatus is shown in FIGURE 3 in which air to be treatedenters an inlet duct 21, passes into a contactor 22 and passesdownwardly through an extended surface 23 therein consisting ofalternate staggered rows of finned tubes 24 with small bafiies 25adjacent alternate rows to prevent by-passing or channeling of the airstream. The air passes from the surface 23 across the surface of a sump26 and upwards through a bed of berl saddles 29 serving as a misteliminator, and thence through an outlet duct 27 and fan 28.

A pump 31 continuously circulates solution from the sump 2.6 throughpipe 32 and valve 33, to deliver the solution through distributingnozzles 34 on to the top of the extended surface 23. The volume ofsolution recirculated will be substantally constant and sufiicient tomaintain the extended surface 23 fully Wetted.

As with the apparatus shown in FIGURE 1 the volume of solution in theapparatus of FIGURE 3 as described will vary by a factor of about 1 to3% over a years time in normal living spaces, while the solutionconcentration varies from about 45% lithium chloride to about 12%lithium chloride. It has been found that the capture efiiciency of thesolution is somewhat improved at lower concentrations and accordingly alevel control 35 may be added to admit fresh water, preferably steamcondensate or dimineralized water sufficient to maintain the solutionmore dilute, say about to 25%. This is especially true where, as in ahospital operating room, .it is necessary to maintain a minimum humidityas well as to control levels of air-borne bacteria, and a concentrationof about 8 to lithium chloride is preferred. This, from FIGURE 2, woulddeliver air at 80 F. having about 135 grains per pound of dry air atequilibrium.

Lithium chloride solutions are not the only known example ofbacteriostatic solutions which, being salt solutions, have no carry outproblems due to vapor pressure losses, and yet are so peculiarlyhygroscopic as to maintain the bacteriostatic properties in equilibriumwith atmospheric air over the complete range of experience in normalliving enclosures. Zinc chloride is bacteriostatic to E. coli and toBacillus subtilis at concentration of 0.1% and is in equilibrium with 70F l1 grain air at 75% solution concentration, and with warm summer airthis solution will dilute to 30%, so zinc chloride theoretically wouldbe an equivalent of lithium chloride. However, corrosion problems ofzinc chloride solutions are so severe as to preclude their use at thepresent time.

Lithium bromide is a third salt which will remain liquid and hyrgoscopicand yet bacteriostatio in normal living atmospheres, its concentrationvarying from 59% in winter to 20% in summer limiting conditions, and itis bacteriostatic to E. coli at 5.8% concentration and to Bacillussubtilis at 5.5%. Due to special problems with this salt primarily itstendency to impart odor and color of bromine to air treated, its use isnot yet practical.

Except for the primary drawbacks of corrosiveness for zinc chloride andof odor for lithium bromide, there are three compounds which are liquid,hygroscopic, and bacteriostatic solutions in equilibrium with air foundin normal living enclosures and thus suitable for use as taught herein,lithium chloride being preferred as indicated.

Comparisons of solution of lithium chloride, lithium bromide and Zincchloride for their bacteriostatic properties show that where E. coli wasused as a test bacteria, the concentrations at which growth wasinhibited were:

Percent Lithium chloride 2.8 Lithium bromide 5.8 Zinc chloride 0.1

While the above tests were made on pure solutions from laboratoryreagents, commercial lithium chloride may in fact contain smallimpurities of each of the other two; in fact where lithium chloridesolution is used in a galvanized container, or in contact with a zincsurface, zinc chloride is formed by corrosive action, unless controlled,and several percent of zinc chloride may be present in the lithiumchloride solution, as much as 8% in a 45% solution may be found incommercial air conditioners.

While equilibrium data must be adjusted for such impurities, thebacteriostatic effects are not reduced and in fact are improved.

While small impurities of calcium chloride or sodium chloride may bepresent, they are subject to salt-ing out when used alone, and are notso effective in control of air-borne micro-organisms.

Saturated solutions of LiCl, NaCl, and CaCl were placed in beakers in anair stream, and the NaCl solution dried in 5 minutes, the calciumchloride solution dried overnight in less than 16 hours, and the lithiumchloride solution reached constant volume in a few hours and remained sofor over an entire weekend. In other test apparatus such a solution wascontinuously recirculated in an air stream in apparatus according toFIGURE 3 for over three months with no measurable loss of salt solutionvolume.

As has been noted above, the 'efiiciency of contactor apparatus is afunction of impingement, or capture, efiiciency of the wetted surface inthe unit. In a particular series of tests using the techinques describedin volume LVI, Number 5, of the Ohio Journal of Science (September 1956issue), pages 305 to 313, for the tall-glass impinger noted on page 306therein, which in the commercial version is accurate to 99% reproducableresults, an air conditioning unit of the type shown in FIGURE 3 wastested with a lithium chloride solution of about 10%. The apparatus wastested at an air flow rating of 500 cubic feet per minute per squarefoot of eifective air flow area through each row of finned coils. Whenso tested with a contact surface consisting of 7 rows of finned tubes39.2% of air-borne microorganisms passed through the unit. Uponretesting the unit at the same conditions except with 12 rows of finnedtubes, 14.4% passed through the unit, and with 18 rows of tubes only2.5% passed through.

The above data are plotted in FIGURE 4 showing the fraction passed vsthe square of the number of rows of tubes, and fits a straight linecurve. This may be written:

f=the fraction passed through the unit; a=the fraction absorbed with norows of tubes;

x=the number of rows of tubes, or the thickness of the wetted contactsurface; and m=a constant.

The capture efficiency C.E. of the unit may thus be defined:

It has been found that the mechanism of capture of air-bornemicro-organisms clues not strictly parallel that of absorption of watervapor by a hygroscopic solution. In tests run on the same apparatus atnear full, 95% air flow and at reduced, 23% air flow, the captureefficiency dropped from 96.3% to 93.3%, while the approach to humidityequilibrium with the hygroscopic solution increased from 90.0% to 97.0%.The improved approach to humidity equilibrium is explained by theincreased contact time, the wetted surface remaining constant, but thechange in capture efiiciency requires other explanation.

Considering air-borne micro-organisms as solid particles, it ispostulated that an impingement mechanism is involved in the capture ofbacteria etc., by the solution. Thus a change of direction of the airstream tends to throw the organisms against the solution, where oncecaptured they are not released to the air stream in a culturable form.if the data showed improved capture of larger organisms, thisimpingement effect would be considered as controlling, but in testsSerratia marcesens, having an average size reported as about 0.25micron, were captured equally well as larger bacteria. Thus there seemsnot to be a pronounced separation of sizes captured at least in the sizerange of bacteria, molds and the like but several direction changes areimportant.

it is believed, however, that surface tension of the solution, orwettabllity of particles to be captured have some bearing on theefficiency of the unit. In tests on the natural flow of air in an olficespace, a test unit having 18 rows of wetted surface finned tubes wasoperated with the lithium chloride solution maintained at 21% and 45% insuccessive runs. Capture GPfiCiBHClC-IS of 97.5%, 96.6% and 96.2%respectively were obtained, with equal proportions of all microorganismsfound in the entering air stream being captured. The fraction passed bythe 45% solution was .038, or 52% more than the fraction (.025) passedby the 10% solution. The im pingement mechanism thus appears to operateon laws of probability, and not selectively for some organisms asagainst others.

While it was once considered by many to be essential to regenerate thelithium chloride solution to maintain its bacteriostatic action, a runof an equilibrium solution thereof averaging about 20% lithium chloridesalt in an oflice summer-cooled atmosphere having a variety of airbornemicro-organisms, including pathogens, the capture efiiciency of the testapparatus was 97.1% for a month of continuous operation, and during anadditional two months additions of organisms were made directly to thesolution to simulate the capture for 32 months, and during and at theend of the period the capture efiiciency remained at 97%, plus or minusless than 1%. At the end of the run, solids from the solution sump werecollected by filtration and washing, and repeated attempts to culturewere fruitless.

it is now possible, with the apparatus and method herein described, tospecify and maintain the bacteria, etc., count allowed in a room belowmeasured levels by either fresh air passed through lithium chloridewasher, or by recirculating air from the room therethrough, and to do socontinuously with an unregenerated lithium chloride solution, withimpurities of lithium bromide or zinc chloride if present. For thepurpose of the following claims, the term lithium chloride shallaccordingly mean lithium chloride with or without minor impurities,including zinc chloride and lithium bromide.

I claim:

1. A method for purifying air which consists in circulating a stream ofa hygroscopic salt solution selected from the group consisting oflithium chloride, lithium bromide and zinc chloride solutions from abody thereof into con tact with a surface and back to the body of thesolution, the surface being wetted by the solution, circulating a streamof air from a source into impinging relationship with the wetted surfacefor purification, and delivering the resulting purified air into anenclosed space, whereby variations in moisture content in the air fromthe source cause variations in concentration of the salt solution.

2. A method for purifying air which consists in circulating a stream oflithium chloride solution from a body thereof into contact with asurface and back to the body of the solution, the surface being wettedby the solution, circulating a stream of air from a source intoimpinging relationship with the wetted surface for purification,delivering the resulting purified air into an enclosed space, and addingwater to the circulated lithium chloride solution to maintain theconcentration thereof substantially constant.

3. A method for purifying air which consists in circulating a stream oflithium chloride solution from a body thereof into contact with asurface and back to the body of the solution, the surface being wettedby the solution, circulating a stream of air from a source intoimpinging relationship with the wetted surface for purification, anddelivering the resulting purified air into an enclosed space, wherebyvariations in moisture content in the air from the source causevariations in concentration of the lithium chloride solution.

4. A method for purifying air which consists in circulating a stream oflithium chloride solution from a body thereof into contact with asurface and back to the body of the solution, the surface being wettedby the solution, circulating a stream of air from a source intoimpinging relationship with the wetted surface for purification,delivering the resulting purified air into an enclosed space, and addingwater to the circulated lithium chloride solution as required tomaintain the concentration thereof between about 5 percent and about 25percent.

5. A method for purifying air which consists in circulating a stream ofa hygroscopic salt solution selected from the group consisting oflithium chloride, lithium bromide and zinc chloride solutions from abody thereof into contact with a surface and back to the body of thesolution, the surface being wetted by the solution, circulating a streamof air from a source into impinging relationship with the wetted surfacefor purification, delivering the resulting purified air into an enclosedspace, and adding water to the circulated salt solution as required tomaintain the concentration thereof between about 5 percent and about 25percent.

References Cited in the file of this patent UNITED STATES PATENTS1,173,497 Farley Feb. 29, 1916 1,992,177 Bichowsky Feb. 26, 19352,129,275 Hartzell Sept. 6, 1938 2,171,400 Lyon Aug. 29, 1939 2,205,831Hartman June 25, 1940 2,230,088 Podbielniak Jan. 28, 1941 2,683,074Kuehner July 6, 1954 FOREIGN PATENTS 3,170 Great Britain Aug. 3, 1880OTHER REFERENCES McCulloch: Disinfection and Sterilization, 1st ed.,1936, pp. 237-240.

Carswell: Ind. and Eng. Chem., vol. 29, No. 1, January 1937, pp. -89.

Chemical and Metallurgical Engineering 47(5), p. 303, May 1940(reproduced in US. Patent No. 2,876,507).

1. A METHOD FOR PURIFYING AIR WHICH CONSISTS IN CIRCULATING A STREAM OF A HYGROSCOPIC SALT SOLUTION SELECTED FROM THE GROUP CONSISTING OF LITHIUM CHLORIDE, LITHIUM BROMIDE AND ZINC CHLORIDE SOLUTIONS FROM A BODY THEREOF INTO CONTACT WITH A SURFACE AND BACK TO THE BODY OF THE SOLUTION, THE SURFACE BEING WETTED BY THE SOLUTION, CIRCULATING A STRAM OF AIR FROM A SOURCE INTO IMPINGING RELATIONSHIP WITH THE WETTED SURFACE FOR PURIFICATION, AND DELIVERING THE RESULTING PURIFIED AIR INTO AN ENCLOSED SPACE, WHEREBY VARIATIONS IN MOISTURE CONTENT IN THE AIR FROM THE SOURCE CAUSE VARIATIONS IN CONCENTRATION OF THE SALT SOLUTION. 